2015 Volume 55 Issue 8 Pages 1721-1729
In this study, plain C–Mn steels with different ferrite grain size were obtained through various thermo-mechanical processing. Fast cooling suppressed ferrite transformation at high temperature, but promoted ferrite nucleation at low temperature and restricted ferrite grain growth. The microstructure and mechanical properties were governed by the combined effects of cooling rate and cooling temperature. A new thermo-mechanical controlled processing (TMCP) based on ultra fast cooling (UFC) technology was adopted to increase the strength of plain C–Mn steel strips. The new TMCP approach involving UFC enabled significantly finer grain size to be obtained in relation to conventional TMCP. The grain size of Fe-0.17C-0.33Mn steel strips obtained by the new TMCP was refined to ~3–4 µm, and both yield strength and tensile strength were increased by 60–100 MPa over the conventional TMCP.
The development of environment-friendly materials and technology has attracted the attention of researchers due to environmental pollution and excessive resource consumption. Exploiting potential properties of conventional steel products, for example C–Mn steels, is one of the efficient approaches to reduce emission and save resources. The strength, toughness, and fatigue properties of C–Mn steels can be significantly improved by grain refinement. Additionally, grain refinement improves weldability and segregation in hot rolled steel strips because of less carbon and reduced alloying elements in a given grade of steel.1,2,3,4,5,6)
Although extremely fine ferrite grain size of ~1 μm can be obtained at the laboratory scale, it is difficult to obtain on an industrial scale.5,6) The common route to refine the ferrite grain size of C–Mn steel is accelerated cooling after controlled rolling.7,8) The accumulation of strain in austenite obtained during hot rolling in the non-recrystallization region is helpful in the transformation of austenite to fine ferrite. The accelerated cooling provides significant driving force and increases nucleation rate, thereby refining the grain size. Microalloying elements are generally added to restrict growth of austenite and enlarge the non-recrystallization region. Grain refinement in plain C–Mn steel without microalloying elements is not popular because of large grain size of austenite and small non-recrystallization region, which may also result in variation in mechanical properties in the hot strip.9) Another grain refinement method is strain induced dynamic transformation, which is effective in plain C–Mn steels.10,11,12,13) However, the large reduction per pass and low rolling temperature during rolling are barriers in industrial production. Ultra fast cooling after hot rolling is a new and effective method to refine the ferrite grains.14,15) This approach does not require severe deformation. It was suggested via quench dilatometry studies that rapid cooling can refine the ferrite grain size and increase the hardness of plain C–Mn steels. The grain size of Fe-0.11C-0.45Mn steel was reduced from 10–11 μm to 4–5 μm with increase in cooling rate from 1°C/s to 600°C/s.16)
Previous studies on the effect of UFC on microstructural evolution in plain C–Mn steel were mainly studied via simulated experiments or at the laboratory scale, and only some of them involved the effect of cooling temperature.16,17,18) The present study focuses on the microstructure and mechanical properties of plain C–Mn steels. The combined effects of cooling rate and cooling temperature on microstructure and mechanical properties were studied through simulation of the TMCP process, in conjunction with recently developed ultra fast cooling in our laboratory, and compared with low cooling and fast cooling condition.
The chemical composition of the experimental steel is presented in Table 1. The steel plates were heated to 1200°C for 2 h, hot rolled to 11 mm thickness on two-high 450 mm experimental hot rolling mill, and air cooled to room temperature. For simulation experiments, cylindrical specimens of 8 mm in diameter and 15 mm in length were prepared from the hot rolled plates along the rolling direction. The simulation process was carried out in Gleeble 1500 thermo-mechanical simulator and is presented in Fig. 1. The specimens were reheated to 1200°C for 180 s to ensure complete austenitization and cooled to 850°C for 20 s, follow by hot compression to 50% at a strain rate of 5 s−1. Then, the specimens were cooled to the isothermal temperature in the range of 750°C to 450°C at cooling rates of 2, 10 and 50°C/s, respectively. Finally, the specimens were cooled to room temperature at the cooling rate of 2°C/s. In order to simulate the cooling conditions on the run out table, the total time (Δt) of cooling stage and isothermal holding stage was kept constant.
Schematic diagram of thermo-mechanical process simulation.
The processing of plain C–Mn steel strips was performed on 2160 mm hot strip rolling line of Shougang Qian’an Iron and Steel, equipped with UFC system between rolling mill and laminar cooling (LC) system, presented in Fig. 2(a). Figure 2(b) illustrates the thermo-mechanical controlled processing of plain C–Mn steel strips and the corresponding parameters are listed in Table 2. The 230 mm thick continuously cast billets were reheated to 1200–1240°C in an industrial furnace for 2 h, rough rolled and finish rolled at 860°C to final thickness of 3.5 mm (the rolling process was controlled by SIEMENS automatic control system). After hot rolling, the strips were cooled via different cooling path. Steel A was cooled to 610°C by LC system (cooling rate of 40–50°C/s), then cooled to 560°C by air before coiling. Steels B, C, and D were cooled to 670°C, 640°C, and 620°C by UFC system (cooling rate more than 100°C/s), respectively, then cooled to 610°C by LC system, follow by air cooling to 560°C before coiling.
Schematic diagram of plain C–Mn steel strips production process on hot strip rolling line. (a) equipment layout diagram of hot strip rolling line equipped with UFC system; (b) thermo-mechanical process of plain C–Mn steel on hot strip rolling line.
The specimens for optical metallography were prepared from thermal simulation specimens and from the hot rolled strips. They were mechanically ground, polished and etched with 4% nital solution. The microstructures were observed by Leica-DMIRM optical microscope (OM) and FEI Quanta 600 scanning electron microscope (SEM). The grain size was measured using Image Pro-Plus software through the linear intercept method.
The hardness of specimens was determined by universal hardness testing machine (KB 3000 BVRZ) using testload of 10 kgf. Micro hardness measurements of ferrite phase and bainite phase were carried out using FM-700 Vickers hardness testing machine with testload of 10 gf. The tensile tests at room temperature were conducted using SHT 5606 Universal Testing Machine. The tensile samples of gage length of 50 mm and width 25 mm were machined from the hot rolled strips along the rolling direction.
Figure 3 shows the hardness of the thermal simulation specimens. The hardness increased slightly (from 126 HV to 132 HV) with increase in cooling rate from 2°C/s to 50°C/s, when the cooling temperature was 750°C. However, the increment of hardness with cooling rate increased as the cooling temperature was lower than 750°C. For example, when the cooling rate was increased from 2°C/s to 50°C/s, the hardness was increased from 128 HV to 145 HV at the cooling temperature of 650°C, and increased from 130 HV to 155 HV when the cooling temperature was 450°C.
Hardness of plain C–Mn steel at different cooling rate and cooling temperature.
There was an increase in hardness with decrease in cooling temperature. Furthermore, the higher the cooling rate, the greater was the effect. For example, when the cooling temperature was decreased from 750°C to 450°C, the hardness increased from 126 HV to 130 HV at the cooling rate of 2°C/s, and increased from 131 HV to 155 HV at 50°C/s. Therefore, it is implied that the mechanical properties of plain C–Mn steel are simultaneously affected by cooling rate and cooling temperature.
The microstructure of experimental steel at different cooling rates and cooling temperatures are presented in Fig. 4. It is seen that Fig. 4 can be classified into two regions: regions A and B. In region A where the cooling temperatures-cooling rates were 750°C-2°C/s, 750°C-10°C/s, 750°C-50°C/s, 650°C-2°C/s, 550°C-2°C/s and 450°C-2°C/s, the microstructure of specimens consisted of larger size polygonal ferrite and pearlite and was insensitive to change in cooling rate or temperature. The ferrite grain size was nearly constant (12–13.5 μm) in this region (Fig. 5) and is the underlying reason for slight change in hardness with cooling rate and cooling temperature. In region B, the microstructure was strongly influenced by the cooling rate and cooling temperature. When the cooling rate was increased from 2°C/s to 50°C/s at cooling temperature of 650°C, the number of ferrite grains per unit area were increased from ~4527/mm2 to ~11370/mm2. The size of polygonal ferrite grains was significantly decreased with increase in cooling rate (Fig. 5) in region B. When the cooling temperature was reduced from 550–450°C at cooling rate of 50°C/s, polygonal ferrite of small size, bainite, and acicular ferrite were obtained. Bainite and acicular ferrite are expected to enhance the strength of plain C–Mn steel. In region B, with increase in cooling rate and decrease in cooling temperature, the grain size of ferrite was decreased and the volume fraction of bainite was increased, which led to increase in hardness.
Microstructures of plain C–Mn steel at different cooling rate and cooling temperature.
Average diameter of ferrite grains of plain C–Mn steel at different cooling rate and cooling temperature.
The mechanical properties of plain C–Mn steel strips produced via hot strip rolling line are listed in Table 3. Steel A subjected to laminar cooling had the lowest strength, with yield strength of 313 MPa and tensile strength of 421 MPa, but high elongation of 39%. The yield strength, tensile strength and elongation of steel B cooled by ultra fast cooling were 375 MPa, 485 MPa and 37.5%, respectively, which demonstrated that the UFC effectively enhances the strength but does not markedly reduce plasticity, when the UFC temperature is 670°C. Higher strength was obtained by lowering the UFC temperature. When the UFC temperature was reduced to 640°C, the yield strength, tensile strength and elongation were 391 MPa, 497 MPa and 35.5%, respectively. When the UFC temperature was reduced to 620°C, the yield strength and tensile strength was increased to 412 MPa and 513 MPa, respectively, and the elongation decreased to 29%.
Figure 6 shows the microstructure of hot rolled steel strips and the microstructural parameters are listed in Table 4. The microstructure of steel A consisted of polygonal ferrite with average diameter of ~5.8 μm and ~14% lamellar pearlite, which accounts for low strength and large elongation. The UFC after hot rolling promoted ferrite nucleation and restricted growth of ferrite grains, resulting in grain refinement. Listed in Table 4, for steels B, C and D, are the average diameter of ferrite grains of ~3.8 μm with the UFC temperature of 670°C, which decreased to ~3.5 μm and ~3.3 μm, respectively at UFC temperature of 640°C and 620°C. The grain size of Fe-0.17C-0.33Mn steel obtained through UFC on hot strip rolling line breaks the limitation of plain C–Mn steel to be refined through conventional TMCP (~5 μm).13,19) It was reported that the grain size of Fe-0.14C-1.0Mn steel was ~3.4 μm, even though the cooling rate was 300°C/s and cooling temperature was low at 400°C.16,20) Hence, ~3.3 μm can be considered to be the limit of grain size refinement in Fe-0.17C-0.33Mn steel produced through TMCP and involving UFC.
Microstructures of plain C–Mn steel strips produced on hot strip rolling line. (a, c, e, g) optical micrographs of steels A, B, C and D; (b, d, f, h) corresponding SEM micrographs.
It can be seen that there was some bainite in steels B, C and D. When the UFC temperature was 670°C, there was ~13% pearlite and only ~3% bainite. However, the volume fraction of bainite was increased and that of pearlite was decreased with decrease in UFC temperature. The volume fraction of bainite increased to ~12%, while the pearlite content was reduced to ~5% when the UFC temperature was decreased to 620°C. Both refinement of grain size and baintie enhances strength of hot rolled plain C–Mn steel strips. However, the high content of bainite in steel D led to decrease in ductility such that the elongation was reduced to 29%.
To clarify the effect of fast cooling on decomposition of deformed austenite, additional thermal simulation experiments were carried out. The specimens were reheated to 1200°C for 180 s, cooled to 850°C at a cooling rate of 10°C/s and held for 20 s. Then, the specimens were deformed in a compression of 50% strain and 5 s−1 strain rate, cooled to 750°C, 650°C and 450°C, respectively, at cooling rate of 2°C/s or 50°C/s and finally quenched to room temperature.
When the specimens were cooled to 750°C and 650°C at cooling rate of 2°C/s, ~55% and ~85% austenite transformed to ferrite. The quenched microstructure consisted of polygonal ferrite and martensite (austenite not transformed during continuous cooling) (Figs. 7(a) and 7(b)). Thus, it was suggested that the deformed austenite will transform to ferrite on cooling temperature below Ar3 at low cooling rate. Because of high transformation temperature and adequate growth time under these conditions, the polygonal ferrite grain was characterized by larger grain size. When the cooling temperature is lowered to eutectoid temperature, all the untransformed austenite transforms to pearlite. For example, the microstructure of specimen cooled to 450°C, consisted of polygonal ferrite and pearlite.
Quenched microstructures of specimens at different cooling rate and cooling temperature. (a, b, c) cooling temperature of 750°C, 650°C and 450°C, respectively, at cooling rate of 2°C/s; (d, e, f) cooling temperature of 750°C, 650°C and 450°C, respectively, at cooling rate of 50°C/s.
However, when the cooling rate was increased to 50°C/s, there was no ferrite at 750°C (Fig. 7(d)) and only ~25% ferrite at 650°C (Fig. 7(e)). This demonstrated that fast cooling enabled the deformed austenite to rapidly pass through the high temperature region, restraining γ/α transformation at high temperature. The microstructure of specimen cooled to 450°C consisted of polygonal ferrite and bainite, which implied that the fast cooling restricted eutectoid transformation and promoted non-equilibrium transformation, such as bainite transformation. The ferrite grain was refined via fast cooling and the average diameter of ferrite grain was decreased from ~14 μm to 4.5 μm, when the cooling rate was increased from 2°C/s to 50°C/s at cooling temperature of 450°C.
The nucleation and growth of ferrite grains is responsible for the grain size of ferrite. In Fig. 8, the number of ferrite grains per unit area was approximately constant (~5500–6000/mm2) and the average grain diameter was increased from ~9.0 μm to ~14 μm, when the specimens were cooled from 750°C to 450°C at a small cooling rate of 2°C/s. This indicated that the nucleation of ferrite was completed before cooling to 750°C under small cooling rate condition. Subsequently, the ferrite nucleation was not increased but the ferrite grains continued to grow when cooled from 750°C to 450°C. However, there was no ferrite at 750°C when the cooling rate was 50°C/s, and the number of ferrite grains per unit area was increased to ~10000/mm2 and ~17000/mm2, respectively, when cooled to 650°C and 450°C. The amount of ferrite at 450°C increased by more than 150% when the cooling rate increased from 2°C/s to 50°C/s. This implied that nucleation was remarkably suppressed at high temperature but promoted at low temperature under fast cooling condition. Compared with cooling at small cooling rate, fast cooling prevented recovery of deformed austenite at high temperature and became strain hardened on deformation. This deformed austenite was characterized by compressed pie morphology, which means an increase in grain boundary area and intragranular deformation bands. The increase in grain boundary area and intragranular deformation bands are potential nucleation sites. The driving force increases with decrease in cooling temperature. The fast cooling also inhibits release of deformation energy, which may transform into driving force. Both increase in potential nucleation sites and nuclear driving force enhances the nucleate rate. On the other hand, fast cooling also restrains the growth of ferrite grains such that the average grain size does not increase distinctly when cooled from 650°C to 450°C, as shown in Fig. 8. Ultimately, the ferrite grain achieves refinement because of high nucleation rate and small growth rate under the fast cooling condition.
Number of ferrite grains per unit area and average grain diameter at the cooling rate of 2°C/s and 50°C/s.
It is important to understand the effect of cooling rate and cooling temperature on microstructure of plain C–Mn steel to control of mechanical properties during hot rolling. A schematic representation of effects of the two factors on grain size and transformation behavior is presented in Fig. 9.
Schematic of relationship between cooling rate, cooling temperature and microstructure.
In general, the high cooling rate and low cooling temperature promote grain refinement. However, it was found that grain size depends on the combined effects of the two factors, rather than a single factor. Although the cooling temperature is lower, the ferrite grain size is not refined if the cooling temperature is relative high. Similarly, if the cooling temperature is high, the grains are coarse regardless of the high cooling rate. There is a coarse grained region (CGR), where the grain size is large and not sensitive to the cooling rate and cooling temperature (top left region in Fig. 9). The fine grained region (FGR), where the grain size is sensitive to the two factors, is located at lower right of CGR in Fig. 9. In this region, lower cooling temperature and higher cooling rate are responsible for the smaller grain size. However, there is a lower limit of grain size in plain C–Mn steel produced through TMCP regardless of extremely high fast cooling rate and low cooling temperature. The grain size depends on steel chemistry, state of austenite prior to transformation, and cooling condition.21) In our study, the steel chemistry and state of austenite are constant. Therefore, it is considered that the sensitivity of grain size to cooling rate and cooling temperature decreases when the grain size is refined close to the limit grain size, i.e., the grain size remains approximately constant in spite of higher cooling rate and lower cooling temperature. The lower limit of the fine grained region (LFGR) is considered to be located in the lower right in Fig. 9.
The final microstructure depends on both cooling rate and cooling temperature. The final microstructures were in the following order, F+P, F+W, F+B and B from top left to lower right in Fig. 9. Figure 9 summarizes the microstructural characteristics obtained in this study. In specimens obtained via thermo-mechanical process simulation experiments, the cooling temperature and cooling rate covered three regions: intersection region of CGR and F+P, where the microstructure consisted of coarse F and P, corresponding to the specimens under conditions of 750°C-2°C/s, 750°C-10°C/s, 750°C-50°C/s, 650°C-2°C/s, 550°C-2°C/s and 450°C-2°C/s; intersection region of FGR and F+P, where the microstructure was F+P and the grains were refined with decrease in cooling temperature and increase in cooling rate, corresponding to specimens under conditions of 650°C-10°C/s, 650°C-50°C/s, 550°C-10°C/s and 450°C-10°C/s; intersection region of FGR and F+B, where the microstructure consisted of refined F and B, corresponding to the specimens under conditions of 550°C-50°C/s and 450°C-50°C/s. In the case of steel strips produced on hot strip rolling line, the process of steel A is considered to be located at the intersection region of FGR and F+P, and that of steels B, C and D is considered to be located at the intersection region of LFGR and F+P or that of LFGR and F+B.
The strengthening mechanisms of plain C–Mn steel primarily include solution strengthening, fine grain strengthening and phase strengthening. The solution strengthening depends on the chemical composition. The fine grain size strengthening and phase strengthening depend on the final microstructure which is controlled by chemical composition and manufacturing process. To obtain steel with high strength and good plasticity, the ideal cooling rate and cooling temperature should be located in the intersection region of LFGR and F+P in Fig. 9, where the microstructure consists of refined F and P (e.g. steel B). For higher strength, the steel should be cooled to the intersection region of LFGR and F+B or even B (e.g. steel D). However, the plasticity may be deteriorated due to bainite. When the cooling temperature and cooling rate are in the intersection region of CGR and F+P, the steel is most likely to be characterized by excellent plasticity but low strength due to the microstructure consisting of coarse F and P.
In this study, the contribution of solution strengthening can be considered to be constant because of the constant chemical composition of experimental steel. However, microstructure morphologies of the plain C–Mn steels cooled by different processes are variant, which results in the difference in strength. Compared with the steel cooled by LC, ferrite of the steel cooled by UFC is significantly refined. According to Hall-Petch equation: σg=kd−1/2, where σg is the increment of yield strength contributed by fine grain size strengthening (MPa), d is the average grain diameter (mm) and k is the Hall-Petch parameter (17.4 MPa·mm−1/2), the contribution of fine grain size strengthening to yield strength can be obtained.22,23) Figure 10 shows the yield strength, tensile strength and contribution of fine grain size strengthening to yield strength of plain C–Mn steel strips produced on hot strip rolling line. In Fig. 10, the increment in strength of the steel cooled by UFC (ΔYS, ΔTS and Δσg,) is the subtraction with the strength of steel A cooled by LC. It is seen that the increment of yield strength is almost equal to that of tensile strength, which indicates that the effects of UFC on yield strength and tensile strength are equivalent. The increment in fine grain size strengthening (Δσg) of steels B, C and D are ~54 MPa, ~65 MPa and ~75 MPa, respectively. And the increments in yield strength (ΔYS) of steels B, C and D are ~62 MPa, ~78 MPa and ~99 MPa, respectively. It is seen that there are some deviations between Δσg and ΔYS, and the deviation increases from steel B, C and D. The increases in hardness of ferrite and volume fraction of bainite may be responsible for the deviation. Presented in Fig. 6 and Table 4, the ferrite of steel A cooled by LC is polygonal with average micro hardness of 127 HV. However, some quasi-polygonal ferrite and acicular ferrite are presented in the steels B, C and D cooled by UFC, resulting in the increase in hardness of ferrite. The average micro hardness increases with decrease in UFC temperature, e.g., 158 HV at UFC temperature of 670°C but 166 HV at UFC temperature of 620°C. On the other hand, UFC promotes bainite transformation so that the volume fraction of bainite increases from ~2.8% to ~11.6% with decrease in UFC temperature from 670°C to 620°C, although the hardness of bainite is relatively stable (Table 4). Therefore, besides grain refinement, the increases in hardness of ferrite and volume fraction of bainite can increase the strength of plain C–Mn steel.
Yield strength, tensile strength and contribution of fine grain strengthening of plain C–Mn steel strips.
Microstructure and mechanical properties of hot rolled plain C–Mn steel with different cooling rate and cooling temperature were studied through thermo-mechanical process simulation and processing on hot strip rolling line. The conclusions are summarized as follows:
(1) The fast cooling after deformation restrained austenite to ferrite transformation at high temperature, increased ferrite nucleation at relative lower temperature, and suppressed growth of ferrite, leading to grain refinement. The nucleation rate was increased by more than 150% and average grain diameter was reduced from ~13.5 μm to 4.5 μm when the cooling rate was increased from 2°C/s to 50°C/s at cooling temperature of 450°C. Meanwhile, the fast cooling also restrained the eutectoid transformation and promoted non-equilibrium transformation.
(2) The final microstructure and mechanical properties of plain C–Mn steel were governed on the combined effects of cooling rate and cooling temperature. The microstructure and hardness were not sensitive to cooling rate and cooling temperature in the range of 2–50°C/s at 750°C and 450–750°C at 2°C/s. But higher cooling rate and lower cooling temperature led to significant grain refinement and increase in hardness for cooling temperature in the range of 450–650°C at 10–50°C/s.
(3) When the cooling rate was increased from ~44°C/s to greater than 100°C/s after hot rolling on hot strip rolling line, both yield strength and tensile strength of Fe-0.17C-0.33Mn steel were increased by ~60–100 MPa because of fine grain strengthening, in combination with strengthening by bainite. UFC technology with appropriate control of cooling temperature and cooling rate is an economic and efficient approach to improve the mechanical properties of plain C–Mn steel strip.
This work was financially supported by the National Science Foundation of China (No. 51234002) and National Key Technology R&D Program for the 12th five-year plan of China (2012BAF04B01). R. D. K. Misra gratefully acknowledges support from the University of Texas at El Paso, USA.